The modification potentials of capsaicin on the development of pulmonary adenoma in newborn NIH (GP) mice were examined. Mice were given a single subcutaneous injection of 1 mg of benzo(a)pyrene (BP) or 40 micrograms of 9,10-demethyl-1,2-benzanthracene (DMBA) within 24 hours after birth and then 0.01% capsaicin (CAP) in the diet (Groups 1 and 2) for 6 weeks after weaning. Mice of groups 3, 4, 5 and 6 were given capsaicin, BP, DMBA and vehicle alone. All mice were sacrificed at week 9. Capsaicin caused a significant inhibitory effect on the frequency of tumor-bearing mice (BP-treated group) and the mean number of tumor (DMBA-treated group). The inhibitory activity is most profound in the group of female mice given DMBA combined with capsaicin. These results showed that capsaicin has inhibitory potential in the mouse lung tumor development induced by polycyclic aromatic hydrocarbons (BP and DMBA).